The analysis of the three-dimensional crystal structure of L-arabinose-binding protein, an essential component in the high affinity uptake system in E. coli has been extended to 2.4 A resolution. The resulting electron density map was utilized to derive a molecular model. The protein is bilobate, and the arrangement of the two lobes forms a deep and narrow cleft. A study of the binding of sugar to the protein using the technique of small angle X-ray scattering indicates a sugar-induced conformational change, most likely a movement of one domain relative to the other about a "hinge" deep in the base of the cleft separating the two lobes. The cleft contain the sugar binding site.